Principally, there are recuperative or regenerative heat exchanger systems for heat exchange between gaseous media. In the case of recuperative heat exchangers, the flow of the heat-emitting medium is applied directly to one or several flows of heat-absorbing media and the heat is transferred directly through a separating wall. In the case of regenerators, the heat is transferred by means of a heat-storing intermediate medium. Such heat-storing intermediate media are arranged in regenerative heat exchangers in the heat storage chambers of the heat storage body. These frequently concern stacked steel sheet layers which may be enameled when necessary. They are frequently arranged as basket systems which can be inserted as a whole in a heat storage chamber and will fill the same. As an alternative, ceramic bodies or heating surfaces made of plastic are partly used as heat-storing intermediate media.
In the case of known heat exchangers, the heat storage body is arranged either to be fixed or rotatable about its longitudinal axis. The first case is related to as “stator” and the latter case as “rotor”. In a heat exchanger with a rotor, the rotor housing including the gas duct connections fastened to the same is arranged in a fixed way, so that the rotor will rotate through the different gas streams. In a heat exchanger with a stator however, rotating gas duct connections, which are so-called rotating hoods, are arranged on both face sides of the stator. In both variants, the different areas of the heat storage body are flowed through in an alternating manner by all existing gas streams.
The heat-emitting gaseous medium flows through the heat storage body from one face side to the other and thus heats the heating elements which are arranged therein in the individual heat storage chambers and which store this heat. Furthermore, one or several heat-absorbing gaseous media flow through the heat storage body, which also occurs from one face side to the other. As a result of the rotation of the rotor or the rotating hoods, the heated heat elements are flowed through by the cold gas streams and thus heat the same.
In the area of power plants, a hot, heat-emitting exhaust gas stream and a cold, heat-absorbing air stream is frequently guided through the heat storage body. This concerns the process of air-preheating. The heated air is then subjected to firing and is then accordingly designated as combustion air. The combustion air heat increased by the heat exchanger substitutes parts of the energy contained in the fuel, thus reducing the fuel quantity required for the firing. As a result, the quantity of CO2 released in the firing is reduced.
Furthermore, the described heat exchangers can also be used for gas preheating. In the case of heat exchangers which are arranged as so-called DeSOX plants, a hot crude gas with high SOx content is cooled for example and a clean gas with low SOx content is heated. In the case of so-called DeNOx plants, a hot clean gas with low NOx content is cooled and a crude gas with high NOx content is heated.
The heat-emitting gas stream and the heat-absorbing gas stream(s) are usually guided to flow against one another through the heat storage body, in line with the countercurrent principle. The heat-absorbing gas is guided out of the heat storage body on the side on which the heat-emitting gas is introduced into the heat storage body. This is known as the hot side of the heat exchanger. Opposite of the same, the cooled heat-emitting gas is ejected and the still cool heat-absorbing gas is injected. This is accordingly the cold side. In the case of a regenerative heat exchanger which is arranged for example for air preheating, it comprises a gas inlet and air outlet on the hot side and a gas outlet and air inlet on the cold side. The exhaust gas flows through an exhaust gas area which extends from the hot to the cold side of the heat exchanger, whereas the combustion air flows through a combustion air area which extends from the cold to the hot side.
The subdivision of the heat exchanger body in heat storage chambers is provided in order to prevent that the different gas streams will mix with each other. Heat-emitting and heat-absorbing gas is guided simultaneously through the different chambers separated from each other. In order to ensure a through-flow or an around-flow of the heat-storing intermediate media located in the heat storage chambers, the heat storage chambers are open on the face sides of the heat storage body.
In order to separate the different gas streams from each other, one or several radial seals are provided on the face sides of the heat storage body. A radial seal is often arranged as a strip or beam and extends orthogonally to the rotational axis or longitudinal axis of the heat exchange body over the diameter of the heat storage body. It is usually arranged in a planar manner and extends through the center point of the heat storage body. It is frequently made of metal or other materials like plastic for example and can be arranged integrally or be made of several parts.
The radial seal can be arranged to be adjustable in the direction of the longitudinal axis of the heat storage body, which means away from the heat storage body or towards the heat storage body. Frequently, the radial seals are arranged in this manner in order to compensate heat-induced deformations of the heat storage body. The sealing gap between the radial seal and the face side of the heat storage body can be kept as small as possible in order to reduce leakages between the various gas streams. Maintaining a minimum sealing gap is necessary in order to ensure the twistability of the heat storage body and radial seal relative to each other.
The radial seal typically consists of two or more sealing arms, with one sealing arm extending substantially from the rotational axis to the outside edge of the heat storage body. The number of sealing arms usually depends on the number of the various existing gas streams. If in a heat exchanger for example which uses a rotor as a heat storage body two gas streams flow through the rotor, two sealing arms each are provided both on the cold as well as the hot side, and three sealing arms in the case of three gas streams, etc. Since the radial seal is arranged in a stationary manner relative to the rotational movement of the rotor, the openings of the heat storage chambers rotate beneath the radial seal. In the case of a complete rotation of the rotor, each point of the face surfaces of the rotor is once beneath and above each sealing arm.
The radial seals are arranged in known regenerative heat exchangers in such a way that one sector wall lies beneath and above a sealing arm in any rotational position, i.e. in any random position of heat storage body and radial seal relative to each other. As a result, the different gas areas such as the combustion air area and the exhaust gas area are always separated by a sector wall extending radially from the rotational axis to the heat storage body edge.
In order to further reduce the leakage between the different gas areas, regenerative heat exchangers have been presented in which the radial seals are arranged in such a way that two sector walls are arranged above and below a rotational arm at least temporarily during the operation of the heat exchanger. In this way, the sectors and thus also the heat storage chambers arranged therein are covered completely once each by the sealing arms during a revolution of the rotor or a revolution of the rotating hood. This helps reduce leakage and improves the efficiency of the heat exchanger. Such a heat exchanger is presented for example in DE 44 20 131 C2, in which at least two adjacent sector walls are arranged beneath a sealing arm even during each rotational position.
Permanent mechanical oscillations are obtained by the continuous closing and opening of the heat storage chambers. They are caused by the different pressure conditions caused by the opening and closing of the heat storage chambers and act in a pulsating manner on the radial seals. This process is called “pumping” of the seals. The intensity of this pumping and thus also the strength of the action on the radial seal depends on the pressure differences present between the various gas streams and the surface area of the seals. Since this process is repeated continuously, the average sealing gap height increases. Moreover, wear and tear of the radial seals and the face surfaces of the heat exchanger body will increase considerably. These factors lead to an increase in leakage. A larger leakage means higher power requirement for the drive of the fans which are required for transporting the flue gases or the air, which shows in a deterioration of the efficiency of the regenerative heat exchanger. In addition to this deterioration, higher leakages lead to an increase in pollutant emissions such as CO2, NOx, SO2, and ashes, which one wishes to keep as low as possible. Moreover, exhaust gas residues can be entrained in the leakage stream which extends beneath the radial seal between the different gas areas, which exhaust gas residues can attack the surfaces of the radial seals, thus further reducing the tightness of the radial seal strips.